Sustainable energy or clean energy is the practice of using energy in a way that "meets the needs of the present without compromising the ability of future generations to meet their own needs."
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Meeting the world's needs for energy in a sustainable way is widely considered to be one of the greatest challenges facing humanity in the 21st century. Worldwide, nearly a billion people lack access to electricity, and around 3 billion people rely on dirty fuels such as wood and animal dung for cooking. Production and consumption of energy cause around 72% of human-caused greenhouse gas emissions and are a major contributor to air pollution, which causes an estimated 7 million deaths per year. Proposed pathways for limiting global warming to 1.5 °C describe rapid implementation of low-emission methods of producing electricity, a shift towards more use of electricity in sectors such as transport, and measures to reduce energy consumption. Achieving this goal will require government policies including carbon pricing and energy-specific policies.
When referring to methods of producing energy, the term "sustainable energy" is often used interchangeably with the term "renewable energy". In general, renewable energy sources such as solar, wind, and hydroelectric energy are widely considered to be sustainable. However, particular renewable energy projects, such as the clearing of forests for production of biofuels, can lead to similar or even worse environmental damage when compared to using fossil fuel energy. There is considerable controversy over whether nuclear energy can be considered sustainable.
Wind and solar energy produced approximately 4.5% of worldwide electricity in 2015. This proportion has grown rapidly and costs are projected to continue falling, but the intermittency of these energy sources presents significant challenges.
The concept of sustainable development was described by the World Commission on Environment and Development in its 1987 book Our Common Future. Its definition of "sustainability", now used widely, was, "Sustainable development should meet the needs of the present without compromising the ability of future generations to meet their own needs."
In its book, the Commission described four key elements of sustainability with respect to energy: the ability to increase the supply of energy to meet growing human needs, energy efficiency and conservation, public health and safety, and "protection of the biosphere and prevention of more localized forms of pollution." Various definitions of sustainable energy have been offered since then which are also based on the three pillars of sustainable development, namely environment, economy, and society.
- Environmental criteria include greenhouse gas emissions, impact on biodiversity, and the production of hazardous waste and toxic emissions.
- Economic criteria include the cost of energy, whether energy is delivered to users with high reliability, and effects on jobs associated with energy production.
- Socio-cultural criteria include the prevention of wars over the energy supply (energy security) and long-term availability of energy.
Providing sustainable energy is widely viewed as one of the greatest challenges facing humanity in the 21st century, both in terms of meeting the needs of the present and in terms of effects on future generations. Bill Gates said in 2011:
Worldwide, nearly a billion people do not have access to electricity, and around 3 billion people rely on dirty fuels for cooking. Air pollution, caused largely by the burning of fuel, kills an estimated 7 million people each year. The United Nations Sustainable Development Goals call for "access to affordable, reliable, sustainable and modern energy for all" by 2030.
Energy production and consumption are major contributors to climate change, being responsible for 72% of annual human-caused greenhouse gas emissions as of 2014. Generation of electricity and heat contributes 31% of human-caused greenhouse gas emissions, use of energy in transportation contributes 15%, and use of energy in manufacturing and construction contributes 12.4%. An additional 5.2% are released through processes associated with fossil fuel production, and 8.4% through various other forms of fuel combustion. As of 2015, 80% of the world's primary energy is produced from fossil fuels.
In developing countries, an estimated 3 billion people rely on traditional cookstoves and open fires to burn biomass or coal for heating and cooking. This practice causes harmful local air pollution and increases danger from fires, resulting in an estimated 4.3 million deaths annually. Additionally, serious local environmental damage, including desertification, can be caused by excessive harvesting of wood and other combustible material. Promoting usage of cleaner fuels and more efficient technologies for cooking is therefore one of the top priorities of the United Nations Sustainable Energy for All initiative. Thus far, efforts to design clean cookstoves that are inexpensive, powered by sustainable energy sources, and acceptable to users have been mostly disappointing.
Proposed pathways for climate change mitigation
Cost–benefit analysis work has been done by a disparate array of specialists and agencies to determine the best path to decarbonizing the energy supply of the world. The IPCC's 2018 Special Report on Global Warming of 1.5 °C says that for limiting warming to 1.5 °C and avoiding the worst effects of climate change, "global net human-caused emissions of CO2 would need to fall by about 45% from 2010 levels by 2030, reaching net zero around 2050." As part of this report, the IPCC's working group on climate change mitigation reviewed a variety of previously-published papers that describe pathways (i.e. scenarios and portfolios of mitigation options) to stabilize the climate system through changes in energy, land use, agriculture, and other areas.
The pathways that are consistent with limiting warning to approximately 1.5 °C describe a rapid transition towards producing electricity through lower-emission methods, and increasing use of electricity instead of other fuels in sectors such as transportation. These pathways have the following characteristics (unless otherwise stated, the following values are the median across all pathways):
- Renewable energy: The proportion of primary energy supplied by renewables increases from 15% in 2020 to 60% in 2050. The proportion of primary energy supplied by biomass increases from 10% to 27%, with effective controls on whether land use is changed in the growing of biomass. The proportion from wind and solar increases from 1.8% to 21%.
- Nuclear energy: The proportion of primary energy supplied by nuclear power increases from 2.1% in 2020 to 4% in 2050. Most pathways describe an increase in use of nuclear power, but some describe a decrease. The reason for the wide range of possibilities is that deployment of nuclear energy "can be constrained by societal preferences."
- Coal and oil: Between 2020 and 2050, the proportion of primary energy from coal declines from 26% to 5%, and the proportion from oil declines from 35% to 13%.
- Natural gas: In most pathways, the proportion of primary energy supplied by natural gas decreases, but in some pathways it increases. Using the median values across all pathways, the proportion of primary energy from natural gas declines from 23% in 2020 to 13% in 2050.
- Carbon capture and storage: Pathways describe more use of carbon capture and storage for bioenergy and fossil fuel energy.
- Electrification: In 2020, around 20% of final energy use is provided by electricity. By 2050, this proportion more than doubles in most pathways.
- Energy conservation: Pathways describe methods to increase energy efficiency and reduce energy demand in all sectors (industry, buildings, and transport). With these measures, pathways show energy usage to remain around the same between 2010 and 2030, and increase slightly by 2050.
Renewable energy sources
When referring to sources of energy, the terms "sustainable energy" and "renewable energy" are often used interchangeably, however particular renewable energy projects sometimes raise significant sustainability concerns. Renewable energy technologies are essential contributors to sustainable energy as they generally contribute to world energy security, reducing dependence on fossil fuel resources, and providing opportunities for mitigating greenhouse gases.
Among sources of renewable energy, hydroelectric plants have the advantages of being long-lived—many existing plants have operated for more than 100 years. Also, hydroelectric plants are clean and have few emissions. Criticisms directed at large-scale hydroelectric plants include: dislocation of people living where the reservoirs are planned, and release of significant amounts of carbon dioxide during construction and flooding of the reservoir.
However, it has been found that high emissions are associated only with shallow reservoirs in warm (tropical) locales, and recent innovations in hydropower turbine technology are enabling efficient development of low-impact run-of-the-river hydroelectricity projects. Generally speaking, hydroelectric plants produce much lower life-cycle emissions than other types of generation.
In 2015, hydropower supplied 16% of the world's electricity, down from a high of nearly 20% in the mid-to late 20th century. It produced 60% of electricity in Canada and nearly 80% Brazil. As of 2017, new hydropower construction has stopped or slowed down since 1980 in most countries except China.
Biomass is biological material derived from living, or recently living organisms. As an energy source, biomass can either be burned to produce heat and to generate electricity, or converted to modern biofuels such as biodiesel and ethanol.
Biomass is extremely versatile and one of the most-used sources of renewable energy. It is available in many countries, which makes it attractive for reducing dependence on imported fossil fuels. If the production of biomass is well-managed, carbon emissions can be significantly offset by the absorption of carbon dioxide by the plants during their lifespans. If the biomass source is agricultural or municipal waste, burning it or converting it into biogas also provides a way to dispose of this waste. Bioenergy production can be combined with carbon capture and storage to create a zero-carbon or negative-carbon system.
As of 2012, wood remains the largest biomass energy source today. If biomass is harvested from crops, such as tree plantations, the cultivation of these crops can displace natural ecosystems, degrade soils, and consume water resources and synthetic fertilizers. In some cases, these impacts can actually result in higher overall carbon emissions compared to using petroleum-based fuels.
Use of farmland for growing fuel can result in less land being available for growing food. Since photosynthesis is inherently inefficient, and crops also require significant amounts of energy to harvest, dry, and transport, the amount of energy produced per unit of land area is very small, in the range of 0.25 W/m2 to 1.2 W/m2. In the United States, corn-based ethanol has replaced less than 10% of motor gasoline use since 2011, but has consumed around 40% of the annual corn harvest in the country. In Malaysia and Indonesia, the clearing of forests to produce palm oil for biodiesel has led to serious social and environmental effects, as these forests are critical carbon sinks and habitats for endangered species. In 2015, annual global production of liquid biofuels was equivalent to 1.8% of the energy extracted from crude oil.
Wind and solar electricity
In 2015, wind power provided approximately 3.5% of the global electricity supply, and solar power provided around 1%. Wind power accounts for approximately 20% of electricity use in Denmark, 9% in Spain, and 7% in Germany. However, it may be difficult to site wind turbines in some areas for aesthetic or environmental reasons. A large wind farm may consist of several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other purposes. A wind farm may also be located offshore. A significant challenge in integrating wind energy into the electrical grid is its high intermittency. Depending on location, some land-based wind turbines generate electricity only 20% to 25% of the time; offshore wind turbines can operate up around 40% of the time.
Solar electricity production uses photovoltaic (PV) cells to convert light into electrical current. Photovoltaic modules can be integrated into buildings or used in photovoltaic power stations connected to the electrical grid. They are especially useful for providing electricity to remote areas.
Most current solar power plants are made from an array of similar units where each unit is continuously adjusted, e.g., with some step motors, so that the light converter stays in focus of the sun light. The cost of focusing light on converters such as high-power solar panels, Stirling engine, etc. can be dramatically decreased with a simple and efficient rope mechanics. In this technique many units are connected with a network of ropes so that pulling two or three ropes is sufficient to keep all light converters simultaneously in focus as the direction of the sun changes. Solar electricity is highly intermittent depending on time of day, season, and cloud cover.
Managing intermittency for wind and solar energy
Solar and wind are intermittent energy sources that supply electricity 10-40% of the time, depending on the weather and the time of day. Most electric grids were constructed for non-intermittent energy sources such as coal-fired power plants. In general, up to around 30% of the energy supplied to an electric grid can be provided by intermittent sources without requiring changes to the grid system.
If intermittent sources make up a larger percentage of the energy supply for a given electric grid, there are several possible approaches to ensuring that electricity generation can meet ongoing demand:
- Using grid energy storage to store excess solar and wind energy and release it as needed. The most commonly-used storage method is pumped-storage hydroelectricity, which is feasible only at locations that are next to a large hill or a deep underground mine. Batteries are being deployed widely. Other storage technologies are used in limited situations.
- Using hydroelectricity or natural gas generation to produce backup power
- Importing electricity from other locations through long-distance transmission lines, such as distributing solar power from the Sahara to Europe.
- Reducing demand for electricity at certain times through energy demand management and use of smart grids.
As of 2019, the cost and logistics of energy storage for large population centres is a significant challenge, although the cost of battery systems has plunged dramatically. For instance, a 2019 study found that for solar and wind energy to meet energy demand for a week of extreme cold in the eastern and midwest United States, energy storage capacity would have to increase from the 11 GW currently in place to 277.9 GW.
Some costs could potentially be reduced by making use of energy storage equipment the consumer buys and not the state. An example is batteries in electric cars that would double as an energy buffer for the electricity grid. Energy storage apparatus' as car batteries are also built with materials that pose a threat to the environment (e.g. Lithium). The combined production of batteries for such a large part of the population would still have environmental concerns.
If renewable sources generate more electricity than the grid uses at a given time, the excess energy could potentially be stored as hydrogen fuel, which has found applications as a vehicle fuel.
Solar heating systems generally consist of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage, and a reservoir or tank for heat storage and subsequent use. The systems may be used to heat domestic hot water, swimming pool water, or for space heating. The heat can also be used for industrial applications or as an energy input for other uses such as cooling equipment. In many climates, a solar heating system can provide a very high percentage (20 to 80%) of domestic hot water energy. Heat can be stored through thermal energy storage technologies. For instance, summer heat can be stored for winter heating. Similar principles are used to store winter cold for summer air conditioning.
Ocean energy refers to the energy carried by ocean waves, tides, salinity, and ocean temperature differences. The movement of water in the world's oceans creates a vast store of kinetic energy, or energy in motion. Some of this energy can be harnessed to generate electricity to power homes, transport and industries. Ocean energy has the potential of providing a substantial amount of new renewable energy around the world. As of 2017, a few small tidal power plants are operating in France and China.
Geothermal energy can be harnessed to for electricity generation and for heating. Technologies in use include dry steam power stations, flash steam power stations and binary cycle power stations. As of 2010, geothermal electricity generation is used in 24 countries, while geothermal heating is in use in 70 countries. International markets grew at an average annual rate of 5 percent over the three years to 2015, and global geothermal power capacity is expected to reach 14.5–17.6 GW by 2020.
Geothermal power is considered to be a sustainable, renewable source of energy because the heat extraction is small compared with the Earth's heat content. The greenhouse gas emissions of geothermal electric stations are on average 45 grams of carbon dioxide per kilowatt-hour of electricity, or less than 5 percent of that of conventional coal-fired plants. As a source of renewable energy for both power and heating, geothermal has the potential to meet 3-5% of global demand by 2050. With economic incentives, it is estimated that by 2100 it will be possible to meet 10% of global demand.
Non-renewable energy sources
Nuclear power plants have been used since the 1950s to produce a steady supply of electricity, without creating local air pollution. In 2012, nuclear power plants in 30 countries generated 11% of global electricity. The IPCC considers nuclear power to be a low-carbon energy source, with lifecycle greenhouse gas emissions (including the mining and processing of uranium), similar to the emissions from renewable energy sources.
There is considerable controversy over whether nuclear power can be considered sustainable, with debates revolving around the risk of nuclear accidents, the cost and construction time needed to build new plants, the generation of radioactive nuclear waste, and the potential for nuclear energy to contribute to nuclear proliferation. These concerns have led to a decrease in the contribution of nuclear energy to the global electricity supply since 1993. At a global level, opposition to nuclear energy stood at 62 percent in 2011. Public support for nuclear energy is often low as a result of safety concerns, however for each unit of energy produced, nuclear energy is far safer than fossil fuel energy.
Traditional environmental groups such as Greenpeace and the Sierra Club are opposed to all use of nuclear power. Individuals who have described nuclear power as a green energy source include early Greenpeace member Patrick Moore, Stewart Brand, George Monbiot, Bill Gates and James Lovelock.
Newer nuclear reactor designs are capable of burning nuclear waste until it is no longer (or dramatically less) dangerous, and have design features that greatly minimize the possibility of a nuclear accident. These designs have yet to be commercialized. (See: Molten salt reactor) Some forms of nuclear power can "burn" nuclear waste through a process known as nuclear transmutation, such as an Integral Fast Reactor. Nuclear power plants can be more or less eliminated from their problem of nuclear waste through the use of nuclear reprocessing and newer plants such as fast breeder plants.
For a given unit of energy produced, the greenhouse gas emissions of natural gas are around half the emissions of coal when used to generate electricity, and around two-thirds the emissions of coal when used to produce heat. Natural gas also produces significantly less air pollution than coal. Building gas-fired power plants and gas pipelines is therefore promoted as a way to reduce emissions and phase out coal use, however this practice is controversial. Opponents argue that developing natural gas infrastructure will create decades of technology lock-in for fossil fuels, and that renewables create far less emissions at comparable costs. The life-cycle greenhouse-gas emissions of natural gas are around 40 times the emissions of wind energy.
Moving towards energy sustainability will require changes not only in the way energy is supplied, but in the way it is used, and reducing the amount of energy required to deliver various goods or services is essential. Opportunities for improvement on the demand side of the energy equation are as rich and diverse as those on the supply side, and often offer significant economic benefits.
Efficiency slows down energy demand growth so that rising clean energy supplies can make deep cuts in fossil fuel use. A recent historical analysis has demonstrated that the rate of energy efficiency improvements has generally been outpaced by the rate of growth in energy demand, which is due to continuing economic and population growth. As a result, despite energy efficiency gains, total energy use and related carbon emissions have continued to increase. Thus, given the thermodynamic and practical limits of energy efficiency improvements, slowing the growth in energy demand is essential. However, unless clean energy supplies come online rapidly, slowing demand growth will only begin to reduce total emissions; reducing the carbon content of energy sources is also needed. Any serious vision of a sustainable energy economy thus requires commitments to both renewables and efficiency.
Climate change concerns coupled with Oil price increases since 2003 and increasing government support are driving increasing rates of investment in the sustainable energy industries, according to a trend analysis from the United Nations Environment Programme. According to UNEP, global investment in sustainable energy in 2007 was higher than previous levels, with $148 billion of new money raised in 2007, an increase of 60% over 2006. Total financial transactions in sustainable energy, including acquisition activity, was $204 billion.
Investment flows in 2007 broadened and diversified, making the overall picture one of greater breadth and depth of sustainable energy use. The mainstream capital markets are "now fully receptive to sustainable energy companies, supported by a surge in funds destined for clean energy investment". The increased levels of investment and the fact that much of the capital is coming from more conventional financial actors suggest that sustainable energy options are now becoming mainstream.
Government promotion of sustainable energy
According to the IPCC, both explicit carbon pricing and complementary energy-specific policies are necessary mechanisms to limit global warming to 1.5°C.
Energy-specific programs and regulations have historically been the mainstay of efforts to reduce fossil fuel emissions. Successful cases include the building of nuclear reactors in France in the 1970s and 1980s, and fuel efficiency standards in the United States which conserved billions of barrels of oil. Other examples of energy-specific policies include energy-efficiency requirements in building codes, banning new coal-fired electricity plants, performance standards for electrical appliances, and support for electric vehicle use.
Carbon taxes are an effective way to encourage movement towards a low-carbon economy, while providing a source of revenue that can be used to lower other taxes or to help lower-income households afford higher energy costs. Carbon taxes have encountered strong political pushback in some jurisdictions, whereas energy-specific policies tend to be politically safer. As of 2018, carbon prices are much lower than the actual costs of emissions to society: In 42 OECD countries and territories they averaged US$8 per ton of carbon dioxide emitted, around a quarter of the OECD's estimated actual cost of carbon. Some studies estimate that combining a carbon tax with energy-specific policies would be more cost-effective than a carbon tax alone.
Sustainable energy research
There are numerous organizations within the academic, federal, and commercial sectors conducting large scale advanced research in the field of sustainable energy. Scientific production towards sustainable energy systems is rising exponentially, growing from about 500 English journal papers only about renewable energy in 1992 to almost 9,000 papers in 2011.
Cellulosic ethanol has many benefits over traditional corn based-ethanol. It does not take away or directly conflict with the food supply because it is produced from wood, grasses, or non-edible parts of plants. Moreover, some studies have shown cellulosic ethanol to be potentially more cost effective and economically sustainable than corn-based ethanol. As of 2018, efforts to commercialize production of cellulosic ethanol have been mostly disappointing, but new commercial efforts are continuing.
Algae fuel is an alternative to liquid fossil fuels that uses algae as its source of energy-rich oils. During the biofuel production process algae actually consumes the carbon dioxide in the air and turns it into oxygen through photosynthesis. In addition to its projected high yield, algaculture— unlike food crop-based biofuels — does not entail a decrease in food production, since it requires neither farmland nor fresh water. Between 2005 and 2012, dozens of companies attempted to commercialize production of algae fuel. By 2017, however, most efforts had been abandoned or changed to other applications, with only a few remaining.
There are potentially two sources of nuclear power. Fission is used in all current nuclear power plants. Fusion is the reaction that exists in stars, including the sun, and remains impractical for use on Earth, as fusion reactors are not yet available. However nuclear power is controversial politically and scientifically due to concerns about radioactive waste disposal, safety, the risks of a severe accident, and technical and economical problems in dismantling of old power plants.
Thorium is a fissionable material used in thorium-based nuclear power. The thorium fuel cycle claims several potential advantages over a uranium fuel cycle, including greater abundance, superior physical and nuclear properties, better resistance to nuclear weapons proliferation and reduced plutonium and actinide production. Therefore, it is sometimes referred as sustainable.
Currently, photovoltaic (PV) panels only have the ability to convert around 24% of the sunlight that hits them into electricity. At this rate, solar energy still holds many challenges for widespread implementation, but steady progress has been made in reducing manufacturing cost and increasing photovoltaic efficiency. In 2008, researchers at Massachusetts Institute of Technology (MIT) developed a method to store solar energy by using it to produce hydrogen fuel from water. Such research is targeted at addressing the obstacle that solar development faces of storing energy for use during nighttime hours when the sun is not shining. In February 2012, North Carolina-based Semprius Inc., announced that they had developed the world's most efficient solar panel. The company claims that the prototype converts 33.9% of the sunlight that hits it to electricity, more than double the previous high-end conversion rate. Major projects on artificial photosynthesis or solar fuels are also under way in many developed nations.
Large national and regional research projects on artificial photosynthesis are designing nanotechnology-based systems that use solar energy to split water into hydrogen fuel and a proposal has been made for a Global Artificial Photosynthesis project. In 2011, researchers at the Massachusetts Institute of Technology (MIT) developed what they are calling an "Artificial Leaf", which is capable of splitting water into hydrogen and oxygen directly from solar power when dropped into a glass of water. One side of the "Artificial Leaf" produces bubbles of hydrogen, while the other side produces bubbles of oxygen.
Research is ongoing in space-based solar power, a concept in which solar panels are launched into outer space and the energy they capture is transmitted back to Earth as microwaves. A test facility for the technology is being built in China.
Geothermal energy is produced by tapping into the thermal energy created and stored within the earth. It arises from the radioactive decay of an isotope of potassium and other elements found in the Earth's crust. Geothermal energy can be obtained by drilling into the ground, very similar to oil exploration, and then it is carried by a heat-transfer fluid (e.g. water, brine or steam). Geothermal systems that are mainly dominated by water have the potential to provide greater benefits to the system and will generate more power. Within these liquid-dominated systems, there are possible concerns of subsidence and contamination of ground-water resources. Therefore, protection of ground-water resources is necessary in these systems. This means that careful reservoir production and engineering is necessary in liquid-dominated geothermal reservoir systems. Geothermal energy is considered sustainable because that thermal energy is constantly replenished.
Hydrogen can be produced from electricity and water and is a possible energy storage and vehicle fuel with fuel cells. It can be produced when there is momentary surplus of non-dispatchable renewable energy, such as wind and solar, and stored for months to years and distributed with infrastructure built for fossil gas or used to generate electricity again at the same location. There is a substantial energy loss in the full cycle from alternating current to hydrogen and back to alternating current. As it has a low energy to volume content, for vehicles it is easier to use for ships than cars and airplanes. As of 2010, for the same cost a person can travel three times as far using a battery electric vehicle as a hydrogen vehicle. Japanese car manufacturers Toyota and Honda currently offer hydrogen fuel-cell powered passenger vehicles for sale in Japan and the U.S.A. Over $1 billion of federal money has been spent on the research and development of hydrogen and a medium for energy storage in the United States as of 2012. Experimental hydrogen fuel-cell city buses are currently operative in two U.S. transit districts, Alameda/Contra Costa county, California, and in Connecticut. See List of fuel cell vehicles.
Among scientific journals related to the interdisciplinary study of sustainable energy are:
- Bruckner, T.; Bashmakov, I. A.; Mulugetta, Y.; Chum, H.; et al. (2014). "Chapter 7: Energy Systems" (PDF). Intergovernmental Panel on Climate Change Fifth Assessment Report 2014. pp. 511–597.
- Edenhofer, Ottmar (2014). Climate Change 2014: Mitigation of Climate Change : Working Group III contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. New York, NY: Cambridge University Press. ISBN 978-1-107-05821-7. OCLC 892580682.
- IPCC, 2018: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [V. Masson-Delmotte, P. Zhai, H. O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, T. Waterfield (eds.)].
- Kutscher, C.F.; Milford, J.B.; Kreith, F. (2018). Principles of Sustainable Energy Systems, Third Edition. Mechanical and Aerospace Engineering Series. CRC Press. ISBN 978-0-429-93916-7. Retrieved 10 February 2019.
- Smil, Vaclav (2017a). Energy Transitions: Global and National Perspectives. Santa Barbara, California: Praeger, an imprint of ABC-CLIO, LLC. ISBN 978-1-4408-5324-1. OCLC 955778608.
- Smil, Vaclav (2017b). Energy and Civilization : A History. Cambridge, Massachusetts: The MIT Press. ISBN 978-0-262-03577-4. OCLC 959698256.
- Tester, Jefferson (2012). Sustainable Energy : Choosing Among Options. Cambridge, MA: MIT Press. ISBN 978-0-262-01747-3. OCLC 892554374.
- Kutscher, Milford & Kreith 2018.
- Renewable Energy & Efficiency Partnership (August 2004). "Glossary of terms in sustainable energy regulation" (PDF). Retrieved 19 December 2008.
- World Commission on Environment and Development (1987). "Chapter 7: Energy: Choices for Environment and Development". Our Common Future: Report of the World Commission on Environment and Development. Oxford New York: Oxford University Press. ISBN 978-0-19-282080-8. OCLC 15489268.
- James, Paul; Magee, Liam; Scerri, Andy; Steger, Manfred B. (2015). Urban Sustainability in Theory and Practice. London: Routledge.; Liam Magee; Andy Scerri; Paul James; Jaes A. Thom; Lin Padgham; Sarah Hickmott; Hepu Deng; Felicity Cahill (2013). "Reframing social sustainability reporting: Towards an engaged approach". Environment, Development and Sustainability. Springer.
- Evans, Robert L., 1945- (2007). Fueling our future : an introduction to sustainable energy. Cambridge: Cambridge University Press. p. 3. ISBN 9780521865630. OCLC 144595567.CS1 maint: multiple names: authors list (link)
- "The Global Energy Challenge". World Bank Blogs. Retrieved 27 September 2019.
- "Q&A: Bill Gates on the World's Energy Crisis". WIRED. 19 (7). 20 June 2011. Retrieved 2 October 2019.
- Sustainable Energy for All. "Access to Energy". SEforALL. Retrieved 27 September 2019.
- "7 million premature deaths annually linked to air pollution". WHO. 25 March 2014. Retrieved 30 September 2019.
- "Goal 7—Ensure Access to Affordable, Reliable, Sustainable and Modern Energy for All". UN Chronicle. 8 April 2015. Retrieved 27 September 2019.
- "Global Historical Emissions". Climate Watch. Retrieved 28 September 2019.
- World Resources Institute (June 2015). "CAIT Country Greenhouse Gas Emissions: Sources and Methods" (PDF). Retrieved 28 September 2019.
- "Fossil fuel energy consumption (% of total)". World Bank Open Data (in Indonesian). Retrieved 27 September 2019.
- "These cheap, clean stoves were supposed to save millions of lives. What happened?". Washington Post. 29 October 2015. Retrieved 1 March 2019.
- Tester 2012, p. 504.
- Loftus, Peter J.; Cohen, Armond M.; Long, Jane C. S.; Jenkins, Jesse D. (2015). "A critical review of global decarbonization scenarios: what do they tell us about feasibility?" (PDF). Wiley Interdisciplinary Reviews: Climate Change. 6: 93–112. doi:10.1002/wcc.324.
- SR15 Summary for policymakers.
- SR15, C.184.108.40.206.
- SR15, C.220.127.116.11, Table 2.6.1.
- SR15, 18.104.22.168, Table 2.6.1.
- SR15, p. 111.
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- SR15, 2.4.3.
- International Energy Agency (2007). Renewables in global energy supply: An IEA facts sheet, OECD, 34 pages. Archived 12 October 2009 at the Wayback Machine
- Hydroelectric power's dirty secret revealed New Scientist, 24 February 2005.
- Ferris, David (3 November 2011). "The Power of the Dammed: How Small Hydro Could Rescue America's Dumb Dams". Retrieved 4 January 2012.
- Smil 2017b, p. 286.
- Tester 2012, p. 512.
- Retrieved on 12 April 2012.
- Smil 2017a, p. 162.
- Edenhofer 2014, p. 616.
- Smil 2017a, p. 161.
- Lustgarten, Abrahm (20 November 2018). "Palm Oil Was Supposed to Help Save the Planet. Instead It Unleashed a Catastrophe". The New York Times. ISSN 0362-4331. Retrieved 15 May 2019.
- "Global Wind Report Annual Market Update". Gwec.net. Retrieved 21 August 2013.
- Smil 2017b, p. 287.
- "Global wind energy markets continue to boom – 2006 another record year". Retrieved 30 January 2015.
- "European wind power companies growing in U.S. – The Mercury News". 25 April 2007.
- "Concepts for new sustainable energy technologies". Pitb.de. Retrieved 21 August 2013.
- American Physical Society Panel on Public Affairs. "Integrating Renewable Electricity on the Grid" (PDF). American Physical Society. Retrieved 3 June 2019.
- "100% Renewable Energy Needs Lots of Storage. This Polar Vortex Test Showed How Much". InsideClimate News. 20 February 2019. Retrieved 4 June 2019.
- Solar water heating energy.gov
- "Solar assisted air-conditioning of buildings". Archived from the original on 5 November 2012. Retrieved 5 November 2012.
- Carbon Trust, Future Marine Energy. Results of the Marine Energy Challenge: Cost competitiveness and growth of wave and tidal stream energy, January 2006
- Smil 2017b, p. 288.
- Geothermal Energy Association. Geothermal Energy: International Market Update May 2010, p. 4-6.
- Moomaw, W., P. Burgherr, G. Heath, M. Lenzen, J. Nyboer, A. Verbruggen, 2011: Annex II: Methodology. In IPCC: Special Report on Renewable Energy Sources and Climate Change Mitigation (ref. page 10)
- "The International Geothermal Market At a Glance – May 2015" (PDF). GEA—Geothermal Energy Association. May 2015.
- Rybach, Ladislaus (September 2007), "Geothermal Sustainability" (PDF), Geo-Heat Centre Quarterly Bulletin, Klamath Falls, Oregon: Oregon Institute of Technology, 28 (3), pp. 2–7, ISSN 0276-1084, retrieved 9 May 2009
- Bruckner 2014, p. 530.
- "IPCC Working Group III – Mitigation of Climate Change, Annex III: Technology - specific cost and performance parameters" (PDF). IPCC. 2014. p. 7. Retrieved 14 December 2018.
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